Myopia or nearsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point before they reach the retina. Myopia generally occurs because the eyeball or globe is too long or the shape or contour of the cornea is too steep. A minus powered spherical lens may be utilized to correct myopia. Hyperopia or farsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point after they reach or behind the retina. Hyperopia generally occurs because the eyeball or globe is too short or the shape or contour of the cornea is too flat. A plus powered spherical lens may be utilized to correct hyperopia. Astigmatism is an optical or refractive defect in which an individual's vision is blurred due to the inability of the eye to focus a point object into a focused image on the retina. Unlike myopia and/or hyperopia, astigmatism is unrelated to globe size or corneal steepness, but rather it is caused by a non-rotationally symmetric cornea or from the misalignment or positioning of the crystalline lens. The vast majority of astigmatism occurs due to non-rotationally symmetric corneal curvature. A perfect cornea is rotationally symmetric whereas in most individuals with astigmatism, the cornea is not rotationally symmetric. In other words, the cornea is actually more curved or steeper in one direction than another, thereby causing an image to be stretched out rather than focused to a point. A cylindrical lens or toric contact lens, rather than a spherical lens may be utilized to resolve astigmatism.
Corneal astigmatism may be corrected using a hard or rigid gas permeable contact lens. In this case, a fluid or tear lens may exist between the posterior surface of the rigid contact lens and the cornea. This fluid or tear lens follows or assumes the shape of the back surface of the contact lens. Since the index of refraction of the fluid or tear lens is nearly a match for the cornea, the corneal toricity is optically neutralized or reduced. In these cases, a toric lens will not be required. However, rigid gas permeable contact lenses and hard contact lenses are generally less comfortable than soft or hydrogel contact lenses. Since soft or hydrogel contact lenses wrap around the cornea, a fluid lens is generally not found and the tear fluid more closely resembles a thin film. In this case, a toric lens design is required.
A toric lens is an optical element having two different powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, spherical, for correcting myopia or hyperopia and one power, cylinder, for correcting astigmatism built into a single lens. These powers are created with curvatures at different angles which are preferably maintained relative to the eye. Toric lenses may be utilized in eyeglasses, intraocular lenses and contact lenses. The toric lenses used in eyeglasses and intraocular lenses are held fixed relative to the eye thereby always providing optimal vision correction. However, toric contact lenses may tend to rotate on the eye thereby temporarily providing sub-optimal vision correction. Accordingly, currently utilized toric contact lenses also include a mechanism to keep the contact lens relatively stable on the eye when the wearer blinks or looks around. For many high order aberrations, many of which are not rotationally symmetric, positional stability is also required to provide optimal vision correction.
When a toric contact lens is first placed in the eye, it must automatically position or auto-position itself and it then maintains that position over time. However, once the toric contact lens is positioned, it tends to rotate on the eye due to the force exerted on the contact lens by the eyelids during blinking as well as eyelid and tear fluid movement. Maintenance of the on-eye orientation of a toric contact lens is generally accomplished by altering the mechanical characteristics of the toric contact lens. For example, prism stabilization, including decentering of the contact lens' front surface relative to the back surface, thickening of the inferior contact lens periphery, forming depressions or elevations on the contact lens' surface, and truncating the contact lens edge are all methods that have been utilized.
Each of more traditional stabilization techniques have advantages and disadvantages associated therewith. The main disadvantage of these types of designs is that they rely on the interaction of the eyelids and the contact lens' thickness differential to orient the contact lens to the correct location on the wearer's eye. The problem is particularly acute with plus powered toric contact lenses intended for hyperopia.
Astigmatic masking lenses in which the lens vaults over the cornea thereby creating a space between the corneal surface and the lens have also been disclosed. Tear film fills that space and masks the astigmatic properties of the cornea. Current masking lenses which have sufficient stiffness in the central region are either undesirably thick, or are incompatible with the hydrogel materials used in the periphery.
U.S. Pat. No. 4,166,255 discloses a hybrid contact lens based from conventional hydrogels with a rigid central optical area surrounded by or embedded in a relatively soft transparent plastic component with a flexible periphery.
U.S. Pat. No. 4,701,288 disclosed a method of making a hybrid contact lens by sequential ultraviolet photopolymerizations with different reactive mixtures in a mold to make a composite article from which a contact lens may be machined.
U.S. Pat. No. 5,923,397 disclosed a bimodulus contact lens comprising a rigid gas permeable polymeric core and a softer end section attached annularly around the core section.
U.S. Pat. No. 6,579,918 disclosed a method of making a composite contact lens in which one optical component is cast molded around a second optical component, thereby encapsulating the second optical component.
U.S. Pat. No. 8,662,663 disclosed a hybrid soft contact lens including a central portion with a Young's modulus between 435 psi and 14,503 psi and a peripheral portion with a Young's modulus between 29 psi and 435 psi.
However, current masking lenses which have sufficient stiffness in the central region are either undesirably thick, or are incompatible with the hydrogel materials used in the periphery.
Accordingly, it would be advantageous to design contact lenses, including toric contact lenses, that correct for astigmatism as well as possible higher order aberrations caused by corneal geometry with less reliance on specific on-eye orientation and therefore less or no stabilization means.